Without this, returning a 400 gives you just "Bad Request" ... with this, you can return a custom response

README.markdown

If you don't have this in your Web.config and your .NET app tries to return a 400 with custom data, you get this ... nomatter what ...

HTTP/1.1 400 Bad Request
Cache-Control: private
Content-Type: text/html
Server: Microsoft-IIS/7.5
X-AspNetMvc-Version: 3.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Tue, 19 Apr 2011 17:47:46 GMT
Content-Length: 11

Bad Request

NOTE You only get this if you're making requests from a REMOTE IP! If you're testing from localhost, you won't see this. You'll see the custom response you want. Don't be fooled! Test your service from a remote machine!

Ok, so ... with this stuff in your Web.config, you'll get what you want:

HTTP/1.1 400 Bad Request
Cache-Control: private
Content-Type: application/json; charset=utf-8
Server: Microsoft-IIS/7.5
X-AspNetMvc-Version: 3.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Tue, 19 Apr 2011 17:49:02 GMT
Content-Length: 25

{"my":["custom", "json"]}

Web.config.xml

<?xml version="1.0"?>
<configuration>
  <system.webServer>
    <httpErrors existingResponse="PassThrough"></httpErrors>
  </system.webServer>
</configuration>

Thank you sir.

Great find - thanks!

This was exactly what I was looking for. Thanks.

For those like me who need to understand why this works, I did the research. By default IIS will mask your error codes and replace them with default errors. The "PassThrough" option tells IIS to leave your custom errors alone and render them as-is.

Man, this just saved my life. Thanks a lot! 😄

Thank you a lot!

Worked great - thanks!

👍

Awesome! 👍
You only get this if you're making requests from a REMOTE IP!
So, that's why I can't reproduce locally :)

thanks, it helped

Thanks pal.

2020 and this still saves jobs!

Thanks 👍

This works great for returning json, but this will cause problem with 404 redirection - it would not work. Do you have a clue how to have both working?

What if i am getting this error in an Azure function?

Thank you! This was super useful.